Jesus said to Nicodemus: “‘You must be born from above.' The wind blows where it wills, and you can hear the sound it makes, but you do not know where it comes from or where it goes; so it is with everyone who is born of the Spirit.” Nicodemus answered and said to him, ‘How can this happen?” Jesus answered and said to him, “You are the teacher of Israel and you do not understand this? Amen, amen, I say to you, we speak of what we know and we testify to what we have seen, but you people do not accept our testimony. If I tell you about earthly things and you do not believe, how will you believe if I tell you about heavenly things? No one has gone up to heaven except the one who has come down from heaven, the Son of Man. And just as Moses lifted up the serpent in the desert, so must the Son of Man be lifted up, so that everyone who believes in him may have eternal life.”
The expression, "Marvel not," shows that Nicodemus was surprised at His doctrine. Jesus then makes an example from the motion of the wind; "The wind blows where it lists, and you hear the sound thereof, but can not tell whence it comes, and whither it goes: so is every one that is born of the Spirit." That is to say, if no one can restrain the wind from going where it will; much less can the laws of nature, restrain the action of the Spirit.
It is the Holy Spirit Who blows where He lists. It is in His own power to choose, whose heart to visit with in His enlightening grace. Like the wind, no one sees the Spirit; and how do we hear the Spirit's voice? When one filled with the Holy Spirit is present with you and speaks to you. The Psalm sounds, the Gospel sounds, the divine word sounds; it is the sound of the Spirit. This means that the Holy Spirit is invisibly present in the Word and Sacrament, to accomplish our birth.
After such clear evidence, Nicodemus persists in thinking in worldly terms. Christ then addresses him with greater severity: "Jesus answered and said to him, 'Are you a master in Israel, and know not these things?'" What are we to think? That our Lord wished to insult this master in Israel? Jesus wished him to be born of the Spirit: and no one can be born of the Spirit unless he is humble. For it is this very humility, which makes us to be born of the Spirit; "For the Lord is near the brokenhearted." (Ps 34:18) Nicodemus, however, was inflated with his eminence as a master, and thought highly of himself because he was a doctor of the Jews. Our Lord then casts down his pride, in order that he may be born of the Spirit.
Catena Aurea (St. Thomas Aquinas)
De Septem Verbis Christi In Cruce (St. Bede the Venerable)
Tractate 12 on the Gospel of John (St. Augustine)
Homily 26 on the Gospel of John (St. John Chrysostom)